Optical fibers are well known in the art and useful for many applications, including laser devices and amplifiers. Basically, an optical fiber comprises an inner core fabricated from a dielectric material having a certain index of refraction and a cladding surrounding the core. The cladding is comprised of a material having a lower index of refraction than the core. As long as the refractive index of the core exceeds that of the cladding, a light beam propagated along the core exhibits total internal reflection, and it is guided along the length of the core. In most practical applications, the refractive indices of the core and cladding differ from each other by only a few percent, which is advantageous for increasing the core diameter for use in single-mode applications.
Designs for optical fibers vary depending upon the application, the desired mode of transmission of the light beam, or the materials used in fabrication. To illustrate, fibers can be fabricated to propagate light of a single mode or multiple modes. Multi-mode fibers typically have a larger core diameter than single-mode fibers to enable a larger mode volume to pass through the fiber, and additional design constraints are posed by multi-mode applications, such as choice of the core and cladding refractive index profiles. The core and cladding can, for example, be of the step index, graded index, depressed clad, or W-type variety, which are characterized by the refractive index of the core relative to the cladding. Multi-core optical fibers also have been fabricated, including multiple cores disposed in arrays within a common cladding, as shown in U.S. Pat. No. 5,627,934 to Muhs, issued May 6, 1997, entitled "Concentric Core Optical Fiber with Multiple-Mode Signal Transmission"; and U.S. Pat. No. 4,000,416 to Goell, issued Dec. 28, 1976, entitled "Multi-Core Optical Communications Fiber," which employ multiple concentric cores primarily for security purposes.
Laser fibers comprise core-pumped and cladding-pumped fibers; that is, the light pumped into the fiber can be channeled directly to the core or it can be pumped into the cladding for reflection into the core. A cross-sectional view of a typical cladding pumped laser fiber is shown in FIG. 1A. As can be seen, typically in laser fibers, a rare-earth doped core 8 is used having a relatively high index of refraction, an example being SiO.sub.2 :GeO.sub.2, with GeO.sub.2 being added to raise the refractive index. The core is surrounded by a pure silica inner cladding 9, having a non-circular circumference, which typically is polygonal, and the cladding is coated with a polymer outer layer 10. Due to the polygonal shape of the inner cladding, light pumped into the total cross-sectional area of the fiber is reflected into the core to provide a laser. The non-circular shape of the inner cladding causes ray distortion and mode mixing of the light so that the light rays are directed to the core, i.e., if a circular inner cladding were used, pump modes of helical paths would be trapped in a path around the symmetrical cladding and not reach the core-to-cladding interface, as further explained in U.S. application Ser. No. 08/856,708, entitled "Cladding-Pumped Fiber Structure," filed May 15, 1997, by D. J. DiGiovanni (the inventor herein), pending, as a continuation of application Ser. No. 08/561,682, filed Nov. 22, 1995, assigned to Lucent Technologies, Inc. (the assignee herein), now abandoned, and incorporated herein by reference. Thus, the circular inner cladding is not effective for the cladding-pumped laser fiber.
The function of the polymer outer layer 10 in this instance (FIG. 1A) is both optical and mechanical, that is, as a protective coating it prevents niches or bends in the cladding from adversely impacting on its optical properties. However, it is generally known that a large difference in the index of refraction between the polymer outer layer 10 and inner cladding 9 is needed to ensure that light rays are contained within the fiber and reflected into the core. For example, the inner cladding 9 is typically fabricated with pure silica with a refractive index of about 1.45, and the refractive index of the polymer is typically about 1.38 or less. It has been understood that divergence in the refractive indices is necessary for use of the fiber as a cladding-pumped laser.
While there are functional benefits to be derived from using asymmetrical features and, in particular, a non-circular cladding, the manufacture of such fibers is complicated because it is difficult to achieve a non-circular cross-section for the inner cladding 9 while maintaining low-loss characteristics. Additionally, fibers having non-circular inner claddings present handling problems and difficulties associated with incorporation of the fibers into laser or amplifier devices.
Polarization-maintaining fibers also rely upon asymmetrical features, which likewise have presented manufacturing and handling difficulties. For example, difficulties associated with manufacturing a polarization-maintaining fiber having an elliptical-shaped core are noted in U.S. Pat. No. 5,482,525, to Kajioka et al., issued Jan. 9, 1996, entitled "Method of Producing Elliptic-Core Type Polarization-Maintaining Optical Fiber." In a polarization-maintaining fiber, two orthogonally polarized modes propagate down a fiber and asymmetrical features are used to maintain the polarization. Typically, this has been accomplished in a single-mode transmission fiber.
For example, shown below in FIGS. 1B to 1F are examples of prior art polarization-maintaining single mode fibers, all of which rely upon a cross-section for the fiber which is in some way asymmetrical to maintain polarization. FIG. 1B shows a fiber having an elliptical-shaped core 12, which is effective with regard to polarization properties, with a relatively short beat length, the beat length being a measure of the effectiveness of the polarization-maintaining properties of the fiber. As the light waves travel along the length of the fiber, their phase relationships change, affecting the polarization state of the modes. With polarization-maintaining fibers, after a certain length called the beat length, the original polarization will recur. Fibers having a large birefringence are known as "Hi-Bi" fibers. They desirably have relatively short beat lengths and are capable of maintaining a linear polarization state over large distances.
In FIGS. 1C to 1D, there is shown an asymmetric region outside the core, the asymmetric region being stress-inducing regions 13 disposed within the inner cladding in FIGS. 1C and 1D. The commonly-used polarization-maintaining fiber of FIG. 1C is called the bow tie type, which is usually fabricated with a gas phase etching process, and FIG. 1D is called the panda type, which incorporates borosilicate rods in the cladding. In FIGS. 1E and 1F, the outer cladding 14 is rectangular-shaped. These are identified as being of the flat-cladding type, involving a non-circular outer cladding diameter, which have been contemplated for improving coupling properties of optical integrated circuits.
As noted, such polarization-maintaining fibers typically have been used for single-mode transmission fiber and present manufacturing and/or handling difficulties. Accordingly, there is a need for a laser fiber having a circular inner cladding and a polarization-maintaining fiber useful in a laser or amplifier having index modulation within a circular inner cladding. The invention addresses these needs. Further advantages may appear more fully upon consideration description below.